A major concern in the operation of machine tools is the maintenance of the cutting tools therein. It is important to immediately identify tools which have become worn, chipped or broken. Historically, on relatively simple machine tools, the operator would continue using a cutting tool until it was visibly worn out or broken. The "use until failure" method results in the production of workpieces having cuts of varying precision as the tool becomes dull. More recently, as machine tools have become more sophisticated and complex, such as with the use of multiple, remotely operated cutting tools, it has become increasingly important to continuously monitor all of the cutting tools, for if a broken tool is undetected, the continuation of work may damage the workpiece and the other cutting tools as well as the machine tool itself. Additionally, an unanticipated tool failure will necessitate a machine shutdown which can adversely effect an entire manufacturing facility.
Preventive maintenance programs which involve the periodic replacement of cutting tools before they wear out have been adopted. Although such programs have the advantage of permitting predicted down-time, such as in the evening or between work shifts, it introduces waste in that tools with some useful life remaining are discarded. Accordingly, to maximize machine tool efficiencies and to minimize waste, it is important to monitor individual cutting tools and to accurately determine the amount of wear of each as well as to immediately signal the operator and/or shut-down the machine should a catastrophic failure occur.
In the prior art, the subject of broken, chipped or worn tool detection has been approached in many ways. Most techniques involve the mechanical monitoring of a cutting tool or its associated workpiece. Wear detection has been accomplished by vibration analysis, receiving an electrical signal from a switch embedded in the cutting tool at a predetermined wear point and releasing a highly visible or odor producing substance such as die or gas contained within the cutting tool at a predetermined wear point. Although tool breakage is typically detected visually, the prior art has also employed mechanical probes which "feel" the cutting tool, electrical reluctance sensors which detect the movement of tool flutes and light or radio-active sensors which detect the presence or absence of a cutting tool. In the prior art, when a cutting tool tip chips, if not clearly visible to the operator, the tool will typically continue in operation until it fails catastrophically, causes a substantial vibration or otherwise manifests itself. Representative prior art devices are disclosed in U.S. Pat. No. 2,461,164; 3,079,821; 3,627,437; 3,669,557; and 3,994,612.
An additional drawback in prior art devices is that they typically require a sensor probe or the like in the immediate proximity of the cutting tool and workpiece. In highly complex machine tools, particularly those employing closely spaced multiple cutting tools, space is at a premium and such sensors may inhibit the movement or replacement of the cutting tools and/or the workpiece. Additionally, such mechanisms are typically complex and expensive. Finally, they are frequently limited as to function such as only the detection of the presence or absence of a specifically shaped cutting tool or merely the amount of wear of a given tool.